import sys


class node(object):
    def __init__(self, a, l, r):
        self.a = a
        self.l = l
        self.r = r


n = int(raw_input())
s = raw_input().strip(' ')


def make(l, r):
    one = 0
    zer = 0
    for i in range(l, r + 1):
        if s[i] == '0':
            zer += 1
        elif s[i] == '1':
            one += 1
    rt = node(None, None, None)
    if one == 0:
        rt.a = 'B'
    elif zer == 0:
        rt.a = 'I'
    else:
        rt.a = 'F'
    m = (l + r) / 2
    if l == r:
        rt.l = rt.r = None
        return rt
    rt.l = make(l, m)
    rt.r = make(m + 1, r)
    return rt


rt = make(0, 2 ** n - 1)

def post(rt):
    if rt == None:
        return
    post(rt.l)
    post(rt.r)
    sys.stdout.write(rt.a)

post(rt)
print '\n'
